The present invention relates to the field of fuel cells. In particular, the present invention relates to electrochemical fuel cells where oxidation and reduction reactions occur causing changes in the free energy and electrical energy.
In an organic and air fuel cell, an organic fuel is oxidized at the anode and an oxygen source is reduced at the cathode. For example, the organic fuel may be methanol, formaldehyde, or formic acid, which is oxidized and reacted with water to produce carbon dioxide and H+ protons (or hydronium ions, H3O+). At the cathode, oxygen in air is reduced and combined with H+ protons (or hydronium ions) to form water. In the context of the invention, protons and hydronium ions are used interchangeably. An exemplary direct organic fuel cell is shown in U.S. Pat. No. 5,599,638 to Surampudi et al.
Surampudi et al. discusses the advantages and disadvantages of known indirect, or reformer fuel cells and direct oxidation fuel cells. It also discusses the problems associated with known electrode structures, and their fabrication and use in fuel cells thereof. For example, electrodes made by conventional methods lacked adequate porosity and wetting during use. Surampudi et al. solves such problems by employing a solid polymer membrane for conducting protons, such as a perfluorinated sulfonic acid polymer membrane. Preferably, the proton-conducting membrane is NAFION(trademark) which is a co-polymer of tetrafluoroethylene and perfluorovinylether sulfonic acid.
In Surampudi et al., the problems associated with prior organic fuel cells (i.e., toxicity, corrosiveness, expensive, poor performance, and complex electrode structure and fabrication) were overcome, in part, by employing the proton-conducting membrane. The primary benefit of using the membrane was that it became practical and feasible to use methanol as the organic fuel. Methanol is desirable because it is relatively non-toxic, non-corrosive, inexpensive and in abundant supply. Methanol also performs well providing a high current density. The use of methanol also allows the electrode structure and fabrication procedure to be simplified producing a more effective electrode.
However, there still exists a need for an organic fuel cell capable of using an alternative oxygen source. In some fuel cell applications, air is absent or available in only limited quantities. For example, submarines and other underwater applications have little to no oxygen available for fuel consumption. Low noise signature, high volume and high specific energy density are also desirable for such underwater applications. High energy fuel cells based on pure compressed hydrogen and oxygen gas are disadvantageous for several reasons. The compressed tanks present safety concerns. The compressed tanks are also heavy and take up a lot of space which are impracticable or undesirable for some applications. Such cells also present other environmental and safety problems.
A need also exists for an oxygen source that is liquid around room temperature so that an electrode for use with liquids may be employed. In some applications, the gas diffusion electrode disclosed in Surampudi et al. may be undesirable. [please describe advantages and disadvantages of liquid v. gas diffusion cell]
As can be appreciated, it would be desirable to provide improved fuel cells that overcome the disadvantages associated with the liquid methanol and oxygen gas fuel cell.
Hence the various general objects of the invention set forth above are achieved. Other objects and advantages of the invention will be apparent from the detailed description set forth below.
One aspect of the present invention is a fuel cell comprising an electrical load, an anode including a catalyst, a cathode including a catalyst, a proton conducting membrane, a methanol source, and an O2 source that is liquid at room temperature. Preferably, the O2 source is hydrogen peroxide. Preferably, the proton conducting membrane is constructed from a perfluorinated sulfonic acid polymer. Preferably, the proton conducting membrane is constructed from a co-polymer of tetrafluoroethylene and perfluorovinylether sulfonic acid. Preferably, the anode includes a platinum-ruthenium catalyst. Preferably, the cathode includes a platinum catalyst. Preferably, the catalysts are particles in supported or unsupported layer configuration on a carbon fiber sheet backing. Preferably, the cathode includes an ion conductor and a catalyst layer comprising the catalyst, an oxygen solvent agent, and a hydrophilic wetting agent, and wherein the anode includes an ion conductor and a catalyst layer comprising the catalyst, and a hydrophilic wetting agent. Preferably, the wetting agent is a compound having perfluorocarbon moieties, and the ion conductor is a carbon fiber sheet backing. Preferably, the wetting agent is a material selected from the group consisting of polytetrafluoroethylene and a co-polymer of tetrafluoroethylene and perfluorovinylether sulfonic acid. Preferably, the oxygen solvent agent is a material selected from the group consisting of a co-polymer of tetrafluoroethylene and perfluorovinylether sulfonic acid and a polytetrafluoroethylene. Preferably, the fuel cell further comprises a separator for maintaining concentration of the hydrogen peroxide in the cathode and a pump for recycling the hydrogen peroxide to the cathode. Preferably, the separator includes a hydrogen peroxide selective membrane. Preferably, the fuel cell further includes an evaporator and condenser for conserving water removed by the separator. Preferably, the fuel cell further comprises a pump for recycling unreacted methanol to the anode. Preferably, the hydrophilic wetting agent is a member selected from the group consisting of a polymeric perfluorosulfonic acid, a polyhydrocarbon sulfonic acid, a polyetherketonesulfonic acid, a polyethersulfone sulfonic acid and a polybenzimidazole. Preferably, the methanol is in a concentration of about 0.25 to 3.0 molar. Preferably, the hydrogen peroxide is in a concentration of about 1-30% volume/volume. Preferably, the hydrogen peroxide is in a concentration of about 3-5% volume/volume.
Another aspect of the invention includes a method of generating electrical energy in a fuel cell comprising the acts of electrochemically reacting methanol in an anode chamber, and electrochemically reacting hydrogen peroxide in a cathode chamber. The method may further include the acts of recycling unreacted methanol to the anode chamber, maintaining an electrochemically sufficient concentration of hydrogen peroxide, and recycling unreacted hydrogen peroxide to the cathode chamber.